Phase Coherence in Wind Data and Simulation

<p>Novel wind turbine designs are deemed acceptable through a simulation-based certification process that involves generating a synthetic wind record and using it as an input to a computer model of the turbine. Naturally, whether the simulation loads reflect the loads that the turbine would actually experience depends on the accuracy of the wind turbine model and, more importantly, on the accuracy of the method used to generate the synthetic wind record. The simulation methods that are commonly used for this purpose are spectral-based and produce Gaussian, stationary random fields. These methods prescribe a power spectral density (PSD) of the wind velocity, which fixes the magnitudes of the Fourier components, then assumes that the Fourier phase angles are independent and uniformly distributed. An inverse Fast Fourier Transform (IFFT) is then used to transform the wind velocity field to the time domain.</p><p>This thesis applies the concept of phase coherence---i.e., Fourier phase angles that are not independent---to the stochastic modeling and simulation of wind velocity fields. Using a large dataset available from the National Wind Technology Center (NWTC), a joint distribution is characterized for the mean wind speed <italic>U</italic>, turbulence &sigma;_u, Kaimal length scale <italic>L</italic>, and a metric for the degree of phase coherence in wind data, <italic>R</italic>. The correlations between these four parameters, the vertical height, and another phase coherence parameter are presented; only <italic>U</italic>, &sigma;_u, and <italic>L</italic> have any significant degree of correlation. The joint distribution is used to generate synthetic wind records, which are then compared with measured data that have the same parameter values. For data with low to medium coherence values, the synthetic records have a similar qualitative appearance to the data. For high levels of phase coherence, the records simulated with the proposed model were qualitatively different from records with the same parameter values due to the variation of the phase difference spread in the spectral domain. Lastly, the importance of correctly modeling phase coherence is demonstrated by using the data and the synthetic records as inputs to a single-degree-of-freedom (SDOF) oscillator and comparing the peak response statistics and damage equivalent loads (DELs).</p>Thesi

The importance of wake meandering on wind turbine fatigue loads in wake

Considering loads when optimizing wind-farm layouts or designing farm-control strategies is important, but the computational cost of using high-fidelity wake models in the loop can be prohibitively high. Using simpler models that consider only the spatial variation of turbulence statistics is a tempting alternative, but the accuracy of these models with respect to the aeroelastic response is not well understood. This paper therefore highlights the effect of replacing wake meandering with spatially varying statistics (“profile functions”) in the inflow to a downstream turbine. Profile functions at different downstream and lateral locations are extracted from a large-eddy simulation with an upstream turbine and compared with two lower-fidelity models: one that prescribes both the mean and standard deviation of the turbulence and one that prescribes only the mean. The aeroelastic response of an NREL 5 MW wind turbine is simulated with the three different wake-model fidelities, and various quantities of interest are compared. The mean values for the power and rotor speed for the medium-and low-fidelity model match well, but the accuracy of the fatigue loads varies greatly depending on the load channel. Prescribing the profile function for the standard deviation is only beneficial for the tower-base fore-aft moment; all other DELs had similar accuracies for both the medium- and low-fidelity models. The paper concludes that blade DELs can be estimated using these simple models with some accuracy, but care should be taken with the load channels related to the shaft torsion and tower-base fore-aft bending moment

Verification of numerical lidars in HAWC2:Analysis of nacelle- and hub-mounted lidars

This report provides a comprehensive assessment of the nacelle and hub-lidar sensors implemented as of HAWC2 13.1. The study originated as part of the CONTINUE project, which led to the implementation of the numerical hub lidar. The main objective of this investigation is to ensure accurate code implementation for both the hub- and nacelle-lidar sensors by verifying that their output aligns with theoretical expectations. Rigorous verifications were performed on various aspects, including the global coordinate positions of the beam focal point, the nominal and weighted line-of-sight wind speed, and the numerical integration for the hub lidar’s weighting function.During the study, several discrepancies were identified in the pre-existing nacelle-lidar implementation. The sensor has thus been updated in HAWC2 13.1 such that it both translates and rotates with the nacelle, as opposed to just translation. Furthermore, the nacelle lidar was corrected to sample the relative velocity observed by the lidar, which is impacted by tower motion, as opposed to the absolute wind velocity.A convergence study on the numerical integration parameters of the hub lidar is also presented using the NREL 5 MW. The study demonstrates that, for the examined conditions, excellent convergence is found for 200 integration points and a half-width of the integration volume equal to 3 times the full width at half-maximum of the lidar. It is recommended that a user conducts their own convergence study using the inflow/lidar parameters relevant for their study

Temporal Trends in Late Preterm and Early Term Birth Rates in 6 High-Income Countries in North America and Europe and Association With Clinician-Initiated Obstetric Interventions

IMPORTANCE: Clinicians have been urged to delay the use of obstetric interventions (eg, labor induction, cesarean delivery) until 39 weeks or later in the absence of maternal or fetal indications for intervention. OBJECTIVE: To describe recent trends in late preterm and early term birth rates in 6 high-income countries and assess association with use of clinician-initiated obstetric interventions. DESIGN: Retrospective analysis of singleton live births from 2006 to the latest available year (ranging from 2010 to 2015) in Canada, Denmark, Finland, Norway, Sweden, and the United States. EXPOSURES: Use of clinician-initiated obstetric intervention (either labor induction or prelabor cesarean delivery) during delivery. MAIN OUTCOMES AND MEASURES: Annual country-specific late preterm (34–36 weeks) and early term (37–38 weeks) birth rates. RESULTS: The study population included 2 415 432 Canadian births in 2006–2014 (4.8% late preterm; 25.3% early term); 305 947 Danish births in 2006–2010 (3.6% late preterm; 18.8% early term); 571 937 Finnish births in 2006–2015 (3.3% late preterm; 16.8% early term); 468 954 Norwegian births in 2006–2013 (3.8% late preterm; 17.2% early term); 737 754 Swedish births in 2006–2012 (3.6% late preterm; 18.7% early term); and 25 788 558 US births in 2006–2014 (6.0% late preterm; 26.9% early term). Late preterm birth rates decreased in Norway (3.9% to 3.5%) and the United States (6.8% to 5.7%). Early term birth rates decreased in Norway (17.6% to 16.8%), Sweden (19.4% to 18.5%), and the United States (30.2% to 24.4%). In the United States, early term birth rates decreased from 33.0% in 2006 to 21.1% in 2014 among births with clinician-initiated obstetric intervention, and from 29.7% in 2006 to 27.1% in 2014 among births without clinician-initiated obstetric intervention. Rates of clinician-initiated obstetric intervention increased among late preterm births in Canada (28.0% to 37.9%), Denmark (22.2% to 25.0%), and Finland (25.1% to 38.5%), and among early term births in Denmark (38.4% to 43.8%) and Finland (29.8% to 40.1%). CONCLUSIONS AND RELEVANCE: Between 2006 and 2014, late preterm and early term birth rates decreased in the United States, and an association was observed between early term birth rates and decreasing clinician-initiated obstetric interventions. Late preterm births also decreased in Norway, and early term births decreased in Norway and Sweden. Clinician-initiated obstetric interventions increased in some countries but no association was found with rates of late preterm or early term birth